diff --git a/app/views/action_text/attachables/_opengraph_embed.html.erb b/app/views/action_text/attachables/_opengraph_embed.html.erb
index f82208a..c5cdd05 100644
--- a/app/views/action_text/attachables/_opengraph_embed.html.erb
+++ b/app/views/action_text/attachables/_opengraph_embed.html.erb
@@ -3,9 +3,9 @@
     <div class="og-embed gap">
       <div class="og-embed__content">
         <div class="og-embed__title">
-          <%= link_to truncate(opengraph_embed.filename, length: 280, omission: "…"), opengraph_embed.href, rel: "noreferrer", target: "_blank" %>
+          <%= link_to_if opengraph_embed.href.present?, truncate(opengraph_embed.filename, length: 280, omission: "…"), opengraph_embed.href, rel: "noreferrer", target: "_blank" %>
         </div>
-        <div class="og-embed__description"><%= truncate(opengraph_embed.caption, length: 560, omission: "…").html_safe %></div>
+        <div class="og-embed__description"><%= truncate(opengraph_embed.caption, length: 560, omission: "…") %></div>
       </div>
       <% if opengraph_embed.url %>
         <div class="og-embed__image">
diff --git a/lib/rails_ext/actiontext_opengraph_embeds.rb b/lib/rails_ext/actiontext_opengraph_embeds.rb
index 916ab10..102e1d8 100644
--- a/lib/rails_ext/actiontext_opengraph_embeds.rb
+++ b/lib/rails_ext/actiontext_opengraph_embeds.rb
@@ -16,12 +16,22 @@ class ActionText::Attachment::OpengraphEmbed
     private
       def attributes_from_node(node)
         {
-          href: node["href"],
-          url: node["url"],
+          href: web_url(node["href"]),
+          url: web_url(node["url"]),
           filename: node["filename"],
           description: node["caption"]
         }
       end
+
+      # A link preview points at what we unfurled, which is always an absolute
+      # http or https URL. Drop anything else the message body asks for, so a
+      # body written by hand can't aim the preview's link or its image at another
+      # scheme or at a path on this Campfire.
+      def web_url(value)
+        value if value.present? && URI.parse(value).is_a?(URI::HTTP)
+      rescue URI::InvalidURIError
+        nil
+      end
   end
 
   attr_accessor :href, :url, :filename, :description
diff --git a/test/controllers/rooms_controller_test.rb b/test/controllers/rooms_controller_test.rb
index 69b2add..2bcae89 100644
--- a/test/controllers/rooms_controller_test.rb
+++ b/test/controllers/rooms_controller_test.rb
@@ -20,6 +20,35 @@ class RoomsControllerTest < ActionDispatch::IntegrationTest
     assert response.cookies[:last_room] = users(:david).rooms.last.id
   end
 
+  test "show renders a link preview written by hand without its off-scheme image and link" do
+    room = rooms(:watercooler)
+    post room_messages_url(room, format: :turbo_stream), params: { message: {
+      body: link_preview_body(href: "javascript:alert(1)", url: "data:image/svg+xml;base64,PHN2Zy8+"),
+      client_message_id: "hand-written-preview" } }
+    assert_response :success
+
+    get room_url(room)
+
+    assert_response :success
+    assert_no_match /javascript:alert/, response.body
+    assert_no_match /data:image\/svg/, response.body
+    assert_match "Free cookies", response.body
+  end
+
+  test "show renders an unfurled link preview" do
+    room = rooms(:watercooler)
+    post room_messages_url(room, format: :turbo_stream), params: { message: {
+      body: link_preview_body(href: "https://example.com/page", url: "https://example.com/image.png"),
+      client_message_id: "unfurled-preview" } }
+    assert_response :success
+
+    get room_url(room)
+
+    assert_response :success
+    assert_match %r{<img src="https://example\.com/image\.png"}, response.body
+    assert_match %r{href="https://example\.com/page"}, response.body
+  end
+
   test "destroy" do
     assert_turbo_stream_broadcasts :rooms, count: 1 do
       assert_difference -> { Room.count }, -1 do
@@ -42,4 +71,10 @@ class RoomsControllerTest < ActionDispatch::IntegrationTest
       delete room_url(rooms(:designers))
     end
   end
+
+  private
+    def link_preview_body(href:, url:)
+      %(<div><action-text-attachment content-type="application/vnd.actiontext.opengraph-embed" ) +
+        %(href="#{href}" url="#{url}" filename="Free cookies" caption="Cookies here"></action-text-attachment></div>)
+    end
 end
diff --git a/test/lib/rails_ext/actiontext_opengraph_embeds_test.rb b/test/lib/rails_ext/actiontext_opengraph_embeds_test.rb
new file mode 100644
index 0000000..15c1481
--- /dev/null
+++ b/test/lib/rails_ext/actiontext_opengraph_embeds_test.rb
@@ -0,0 +1,66 @@
+require "test_helper"
+
+class ActionText::Attachment::OpengraphEmbedTest < ActiveSupport::TestCase
+  test "keeps absolute http and https links and images" do
+    embed = embed_from href: "http://example.com/page", url: "https://example.com/image.png"
+
+    assert_equal "http://example.com/page", embed.href
+    assert_equal "https://example.com/image.png", embed.url
+  end
+
+  test "drops a link and an image that aren't web URLs" do
+    [ "javascript:alert(1)", "data:text/html,pwned", "vbscript:msgbox(1)", "//example.com/image.png",
+      "/rooms/1", "rooms/1", "", "http://exa mple.com/ " ].each do |value|
+      embed = embed_from href: value, url: value
+
+      assert_nil embed.href, "expected #{value.inspect} to be dropped as a link"
+      assert_nil embed.url, "expected #{value.inspect} to be dropped as an image"
+    end
+  end
+
+  test "renders the image and the link when both are web URLs" do
+    html = render_embed href: "https://example.com/page", url: "https://example.com/image.png"
+
+    assert_match %r{<a rel="noreferrer" target="_blank" href="https://example\.com/page">Title</a>}, html
+    assert_match %r{<img src="https://example\.com/image\.png"}, html
+  end
+
+  test "renders no image and no link when neither is a web URL" do
+    html = render_embed href: "javascript:alert(1)", url: "data:image/svg+xml;base64,PHN2Zy8+"
+
+    assert_no_match /javascript:/, html
+    assert_no_match /data:/, html
+    assert_no_match /<img/, html
+    assert_no_match /<a /, html
+    assert_match "Title", html
+  end
+
+  test "renders the title and the description as text" do
+    html = render_embed href: "https://example.com/page", url: "https://example.com/image.png",
+      filename: "<b>Title</b>", caption: "<img src=x onerror=alert(1)>"
+
+    assert_no_match /<b>/, html
+    assert_no_match /<img src=x/, html
+    assert_match "&lt;b&gt;Title&lt;/b&gt;", html
+    assert_match "&lt;img src=x onerror=alert(1)&gt;", html
+  end
+
+  private
+    def attachment_for(href:, url:, filename: "Title", caption: "Description")
+      html = %(<action-text-attachment content-type="application/vnd.actiontext.opengraph-embed" ) +
+        %(href="#{href}" url="#{url}" filename="#{filename}" caption="#{caption}"></action-text-attachment>)
+      node = ActionText::Fragment.wrap(html).find_all(ActionText::Attachment.tag_name).first
+
+      ActionText::Attachment.from_node(node)
+    end
+
+    def embed_from(**attributes)
+      attachment_for(**attributes).attachable
+    end
+
+    def render_embed(**attributes)
+      attachment = attachment_for(**attributes)
+
+      ApplicationController.render partial: attachment.to_partial_path, locals: { opengraph_embed: attachment }
+    end
+end